P1259: CHARACTERIZATION OF VENETOCLAX RESISTANCE IN CELL MODELS OF MANTLE CELL LYMPHOMA
Authors: A. Granau, P. Andersen, T. Jakobsen, L. Sommer Kristensen, J. Bay Mogensen, G. Dias Correa Guldbergsen, K. Dimopoulos, K. Grønbæk
Journal: HemaSphere (2022)

Background: Venetoclax (Ven), a BH3 mimetic inhibiting BCL2, appears to be highly cytotoxic to most Mantle Cell Lymphoma (MCL) cells due to their high BCL2 levels. However, resistance to Ven in both MCL and other lymphomas is being observed. Characterizing underlying mechanisms of acquired Ven resistance can hopefully highlight potential treatment options for patients who develop resistance or prolong the responsive phase of patients receiving Ven. Aims: To investigate and characterize drivers of acquired Ven resistance in cell models of Mantle Cell Lymphoma with an exploratory approach to potentially elucidate drivers of resistance development. Methods: Using two Ven-sensitive MCL cell lines (MAVER1 and MINO) we established Ven resistant cell lines (MAVER1-R and MINO-VR). Changes between the maternal and the VR cells were characterized using full exome and transcriptome sequencing and by AcceSssIble, which investigates changes to CpG methylation and genomic accessibility. BH3 profiling was performed to investigate the degree of dependance on antiapoptotic BCL2 family proteins and mitochondrial priming. Finally, we performed a drug screen which targeted pathways identified through GO analysis of the transcriptomic data, targeting both oncogenic and epigenetic targets. Results: In both MAVER1-VR and MINO-VR we observed an increase in genome-wide CpG methylation concurrently with decreased overall genomic accessibility associated with Ven resistance (figure 1A). However, 2-day pre-treatment with epigenetic drugs (Vorinostat, Tazemetostat, Valemostat and Azacytidine) and 7-day pre-treatment with Tazemetostat + Azacytidine had no effect on Ven sensitivity. Based on RNA sequencing, we observed clustering of the -VR cell lines together, separate from their maternal cell lines indicating common transcriptional changes associated with Ven resistance (figure 1B). GO analysis performed on the differentially expressed genes (DEGs) highlighted PLCb mediated events, JAK-STAT- and PI3K-AKT signaling in development of Ven resistance (figure 1C). Inhibition of JAK1 or JAK2/3 showed no change to Ven sensitivity in neither maternal nor VR cells. Inhibition of mTORC1 with Everolimus abrogated the toxic effect of Ven in the maternal cell lines (figure 1E), whereas TIC-10, an AKT/MEK inhibitor, partially restored Ven sensitivity in MAVER1-VR and MINO-VR cell lines (figure 1D). PLC inhibition didn’t affect Ven sensitivity, however it was more cytotoxic to VR cells than to the maternal cells (figure 1F). BH3 profiling showed a decreased response to the apoptotic activators BIM and PUMA in VR cell lines (figure 1G), indicating lowered mitochondrial priming. Image:Summary/Conclusion: We found a combination of epigenetic and transcriptomic changes associated with Ven resistance development in MCL cell lines; however, the observed epigenetic alterations might not drive Ven resistance alone, since pretreatment with epigenetic drugs did not affect Ven sensitivity. The PI3K-AKT-mTOR pathway appears to be involved in Ven resistance development, since interestingly, mTOR inhibition leads to a decreased response to Ven treatment in maternal cell lines but AKT inhibition leads to an increased response in -VR cell lines. Ven resistance in MINO-VR appears to be driven primarily by decreased mitochondrial priming, whereas MAVER-VR appears to be driven by decreased mitochondrial priming combined with a switch from BCL2 to MCL1 dependence.